Introduction:
The
nervous system is conventionally divided into the central nervous
system (CNS) which consists of the brain and spinal cord, and the
peripheral nervous system (PNS) that consists of neuronal tissues
outside the CNS.
Roles
of the different parts of the nervous system: The
motor (efferent) portion of the nervous system can be divided into
two major subdivisions: autonomic and somatic. The autonomic nervous
system (ANS) is largely independent (autonomous) in that its
activities are not under direct conscious control. It is concerned
primarily with visceral functions such as cardiac output, blood flow
to various organs, and digestion, which are necessary for life.
Evidence is accumulating that the Autonomic Nervous System,
especially the vagus nerve, also influences immune function and some
Central Nervous System functions such as seizure discharge.
On
the other hand, the somatic subdivision is largely concerned with
consciously controlled functions such as movement, respiration, and
posture. Both systems have important afferent (sensory) inputs that
provide information regarding the internal and external environments
and modify motor output through reflex arcs of varying size and
complexity.
The
nervous system has several properties in common with the endocrine
system, which is the other major system for control of human body
function. These include high-level integration in the brain, the
ability to influence processes in distant regions of the body, and
extensive use of negative feedback.
Both
systems use chemicals for the transmission of information.
In the
nervous system, chemical transmission occurs between nerve cells and
between nerve cells and their effector cells. Chemical transmission
takes place through the release of small amounts of transmitter
substances from the nerve terminals into the synaptic cleft. The
transmitter crosses the cleft by diffusion and activates or inhibits
the postsynaptic cell by binding to a specialized receptor molecule.
In a few cases, retrograde transmission may occur from the
postsynaptic cell to the presynaptic neuron terminal and modify its
subsequent activity. By using drugs that mimic or block the actions
of chemical transmitters, we can selectively modify many autonomic
functions.
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